# How does an alpha (AoA) vane work?

The alpha vane (also called AoA vane) is an external probe used to measure the angle of attack. I have been trying to understand how exactly it works, but I can't find any clear explanation or simulation. Is the vane static or dynamic i.e. does it rotate along its central axis?

Given that it has a significant surface area, I think that it would either:

• Rotate because of the force/drag exerted by the airflow, and give an angle of attack proportional or equal to its angle of rotation
• Measure the force being exerted on it via a force sensor embedded in the surface

Is either of these correct?

In short, how does an alpha vane work?

From what I have studied at my instrumentation course, it rotates and reads the angle between the current position and the reference position.

You then have several (3-4) of them around the fuselage because a roll maneuver (for example) will alter the measurements asymmetrically and in this way you can compensate for this effect.

• I didn't realise there were more than two (usually) on aircraft. So, during a roll, if one of the vanes stops working correctly and provides faulty readings, that would provide an incorrect angle of attack to the FMS? (Since as you say, multiple vanes are cumulatively used to estimate the actual alpha). Mar 15, 2014 at 13:22
• @AsheeshR actually during a roll one gets a higher AoA while the other gets a lower one, Mar 15, 2014 at 13:35
• @ratchetfreak Say one freezes, and the other provides a lower reading, then the average? would be lower than the actual angle. So, now, any stall warnings based on alpha that would have been there had both been working, won't come on? Mar 15, 2014 at 13:39
• @AsheeshR It depends, you can also cross-check with the IRU data Mar 15, 2014 at 14:49
• @Federico Wikipedia tells me the ADIRU takes input from the pitot-static system to estimate alpha, which is dependent on the alpha vane. Mar 15, 2014 at 14:54

An AOA vane (like what you have shown) works by aligning itself with the local airflow, like an arrow. The angle to some reference line (normally aircraft fuselage horizontal) is then measured with a potentiometer/RVDT/etc. This is the local angle of attack. Often the local angle of attack is converted to aircraft angle of attack through a calibration curve.

Most transport airplanes have 2 vanes. If either vane shows an angle of attack that is approaching stall, the stall warning will alert the pilot(s) that stall is near. If both AOA vanes are at the stall AOA, a stick pusher will activate to alleviate the aerodynamic stall.

The AOA values are never averaged. Depending on the AOA computer, logic is used to determine if one of the vanes is giving bad data.